Add background-position control

![background-position selected lengths](https://f.cloud.github.com/assets/134745/979423/2aabfdf0-0704-11e3-9b29-e514b912831c.png)
![background-position two keywords](https://f.cloud.github.com/assets/134745/979421/2324495c-0704-11e3-805d-f8c9b3296907.png)
![background-position one keyword one length](https://f.cloud.github.com/assets/134745/979422/278789b4-0704-11e3-8404-fc598bef6846.png)
![background-position keywords](https://f.cloud.github.com/assets/134745/979424/2d8282e2-0704-11e3-96bc-ea1d5e6f7aed.png)